Acinetobacter baumannii is an important nosocomial pathogen that causes a range of diseases, including respiratory and urinary tract infections, meningitis, endocarditis, wound infections, and bacteremia. In fact, A. baumannii is now responsible for up to 20% of all intensive care unit infections in some regions of the world with pneumonia being the most common presentation. The clinical significance of A. baumannii has been propelled by this organism's rapid acquisition of resistance to virtually all antibiotics. The identification of novel targets for therapeutic intervention is critical to our ability to protect he public health from this emerging infectious threat. One promising potential area of therapeutic development involves targeting bacterial access to nutrient metal. This strategy is based on the fact that all bacterial pathogens require nutrient metal in order to colonize their hosts. Despite the fact that a variety of metals are required by bacterial pathogens during growth within vertebrates, iron sequestration is considered to be the primary nutrient that is actively sequestered by the host during the innate immune response to infection. In the present application, we provide evidence that the innate immune factor calprotectin defends against Acinetobacter pneumonia by chelating nutrient zinc (Zn). Using calprotectin as a probe, we have identified a transport system in A. baumannii that competes with calprotectin for Zn and is transcriptionally controlled by a Zn-dependent regulator. Calprotectin is abundant at sites of inflammation and is a known pro- inflammatory molecule that is a ligand for the receptor for advanced glycation end products (RAGE). Despite its clear involvement, the contributions of calprotectin-mediated metal sequestration and RAGE binding to defense against infection have not been thoroughly evaluated. Based on preliminary data described in this application, we hypothesize that calprotectin-mediated Zn sequestration and RAGE binding are critical factors during the host-pathogen interaction. To test this central hypothesis, we propose a series of experiments aimed at understanding the mechanism and pathophysiological consequence of calprotectin-mediated Zn sequestration and RAGE binding during A. baumannii pneumonia. In these studies, we will (i) identify the structural features of calprotectin that enable it to chelae transition metals such as Zn and to bind RAGE, (ii) elucidate the impact of calprotectin and RAGE on A. baumannii pathogenesis, and (iii) determine the impact of CP-mediated Zn chelation on the physiology of A. baumannii. These results will provide fundamental insight into how A. baumannii acquires nutrients in the vertebrate host, and lay the foundation for the creation of peptide therapeutics based on a calprotectin scaffold that inhibit microbial growth through nutrient metal chelation.